Thermal insulation plates or flat, level thermal insulation structures based on precipitated silica which are evacuated and provided with a multi-layer casing are known.
Thus, EP-A 0 190 582 (U.S. Pat. No. 4,636,415) and EP 0 254 993 (U.S. Pat. No. 4,681,788) describe a casing consisting of composite foils which additionally contain a metal foil of e.g. aluminum. These foils should be airtight and watertight.
EP-B 0 164 006 (U.S. Pat. No. 4,636,416) describes heat insulators which contain fine metal oxides and are evacuated. The casing material can be a composite foil with a layer sequence of thermoplastic material/metal foil/thermoplastic material.
Japanese published, unexamined application Sho 62-207 777 (Sept. 12, 1987) describes heat-insulating elements which are produced by filling perlite or other light, porous materials into a container of heat-sealable plastic laminate and then evacuating the interior of this container.
Known heat-sealed, heat-insulating elements consist of plastic laminate with a thickness of 25 .mu.m, a water-vapor permeability of 1.0 g/m.sup.2.d at 38.degree. C. and 90% relative moisture as well as with an oxygen permeability of 2.0 cm.sup.3 /m.sup.2.d at 23.degree. C. and 90% relative humidity. The laminates consist of vinylidene chloride- vinyl chloride copolymer which is vapor-plated on at least one side with a layer of aluminum 100 to 1000 angstroms thick. At least one laminate layer is used.
The known use of metal-coated composite foils has the disadvantage that heat can be conducted parallel to the foil surface. This results, if they are used in insulating materials, in undesirable heat transfer between the cold and the warm sides across the edges of the heat-insulation body. A consequence is an adverse effect on the total thermal conductivity of the heat-insulation body which is not detected in thermal-conductivity measurements according to the absolute single-plate method with protective ring technique according to Kohlrausch (F. Kohlrausch: "Praktische Physik", vol. 1, 22nd edition, B. G. Teubner Verlag, Stuttgart, 1968, p. 375 ff.).
A heat-insulator manufactured according to EP A 0 190 582 using a metallic foil exhibits a thermal conductivity of 8 mW/(m.K) at 23.degree. C., measured in accordance with the above-mentioned method. If a measuring arrangement without protective ring is selected, the thermal conductivity rises as a function of the geometry and size of the insulation article and as a function of the thickness of the metal layer in the casing foil, to values which are in part considerably higher.
The insulating capacity of the entire heat insulator thus also depends to a significant degree on whether the casing foil used in the manufacture is made of metal.
German patent application DE-OS 39 15 170 teaches a shaped article for use as heat insulation which consists of a finely distributed powdery or fibrous substance with a water-absorption capacity of 4 to 50% by weight at 23.degree. C. and 85% relative humidity and of a metal-free casing which surrounds this finely distributed powdery or fibrous substance, exhibiting a water-vapor permeability of 0.1 to 0.5 g/m.sup.2.d at 23.degree. C. and 85% relative humidity and gas permeabilities totalling 0.1 to 0.5 cm.sup.3 /(m.sup.2.d. bar) at 23.degree. C.
These known articles can retain their low thermal conductivity for only approximately 3 years under the specified conditions.
If the heat-insulation bodies are to be used for the insulating of cooling devices, e.g., refrigerators, they must retain the low thermal conductivity for a considerably longer time.
There is, therefore, a problem in manufacturing heat insulators which have low thermal conductivity both transversally and also longitudinally of the composite foil and which retain this low thermal conductivity for a considerably longer time than the above-mentioned 3 years.